The present invention refers to a coffee machine, comprising a fresh water supply, for example a water reservoir, a coffee brewing unit provided with a brewing chamber, means for supplying coffee powder to said brewing chamber, a coffee beverage outlet and a pump for conveying water under pressure from the fresh water reservoir through the brewing chamber containing the coffee powder to the coffee beverage outlet.
More specifically, the invention refers to a so-called xe2x80x9cEspresso Machinexe2x80x9d in which coffee powder contained in a dedicated brewing chamber or in a sieve holder is subjected to hot pressurized water, flowing there through, thus yielding a coffee beverage. In recent espresso coffee machines, meanwhile, there is provided a possibility for the user or operator of the machine to select or adjust the amount of coffee powder to be used for preparing a coffee beverage, as well as to select or adjust the grinding rate of the coffee powder, i.e. the granule size thereof. As a consequence of these possibilities, problems can arise insofar as by varying the above mentioned parameters, the pressure drop in the brewing water flowing through the brewing chamber is influenced as well. It is understood that the pressure drop is increasing the greater the amount of coffee powder is and the finer the coffee beans are ground. In contrast, the pressure drop decreases if the amount of coffee powder in the brewing chamber is decreased and the coffee beans are ground to yield a coarser coffee powder.
Besides these facts, however, the above mentioned pressure drop depends on further parameters that usually cannot be influenced by the user or operator of the coffee machine, for example on the degree of compacting the coffee powder in the brewing chamber. By the above mentioned parameters, it is understood that also the flow rate of the brewing water through the brewing chamber and, thereby, the amount of coffee beverage available at the beverage outlet per time unit is influenced. The result is, depending on the interference of the selected or predetermined parameters, that the coffee beverage flows out of the beverage outlet anyhow between slow and fast without a possibility for the user or operator to control it.
For example, studies performed by the applicant have shown that fundamental differences in habits of the kind of preferred coffee exist between northern countries and southern countries. Generally speaking, in southern countries, a coffee beverage preferably in the form of an xe2x80x9cespressoxe2x80x9d is preferred, while in northern countries a xe2x80x9cstandardxe2x80x9d coffee or a cappuccino is preferred. These studies have further shown that, in southern countries, an espresso coffee beverage is considered to be of high quality if the coffee beverage flows into the cup but very slowly, while a quick preparation of a standard coffee is preferred in northern countries, since the amount of coffee to be prepared in the case of a standard coffee is much higher than the one in the case of an espresso coffee. It is understood that the dwell time of the brewing water in the brewing chamber is longer if the flow rate, i.e. the amount of brewing water flowing through the brewing chamber per time unit, is low than is the case of a high flow rate; thus, selecting a low flow rate results in a more effective extraction of the coffee powder. Thereby, it can also be explained why the flow rate has an influence on the taste of the prepared coffee beverage. However, the flow rate of the brewing water flowing through the compacted coffee powder received in the brewing chamber can be influenced only indirectly by the user or operator of a coffee machine known in the art, for example by adjusting the degree of grinding of the coffee beans and the granule size of the coffee powder, respectively.
Known in the prior art are coffee machines comprising a valve assembly with a design to increase the pressure in the brewing chamber, resulting in the fact that a foam is created appearing on the top surface of the coffee beverage dispensed into a coffee cup. However, such known valve assemblies, as far as the flow rate of the brewing water flowing through the compacted coffee powder received in the brewing chamber is concerned, have only an effect insofar as they increase the back pressure in the brewing chamber and, thereby, decrease the flow rate of the brewing water in any case.
Such a valve assembly is disclosed, for example in the document DE 30 35 157. In that document, a brewing head for an espresso coffee machine is described which is provided with a sieve insert for receiving the coffee powder. At the outlet of that sieve insert, a spring biased check valve is provided by means of which the pressure in the brewing head is increased to such an extent that a foaming coffee beverage is dispensed.
Moreover, the document EP 0 542 045 discloses a coffee brewing device in which a valve is located at the outlet of the brewing chamber. The valve body member thereof is biased against the flow direction of the coffee beverage. The valve body member can be moved, against the force of the bias spring, under the influence of the coffee beverage escaping from the brewing chamber. Again, with that design, a back pressure in the region of appr. 1 bar shall be created for providing a coffee beverage having a durable foam on its top.
The document CH 668 543 discloses an espresso coffee machine of the kind referred to herein, having a spring-biased ball valve located at the outlet of the brewing chamber. Again, by means of that ball valve, a coffee beverage having a durable foam on its top shall be created.
Finally, the document EP 0 726 053 discloses an apparatus for dispensing a coffee beverage, having a filter container for receiving the coffee powder and an outlet channel connected to that filter container and comprising a check valve member. That check valve member comprises a valve body member kept in contact with a valve seat by an elastic means. Between the valve body member and the valve seat, a closed gap is formed in which the solid particles released from the coffee powder can collect in order to form a plug. As soon as a predetermined pressure is reached in the filter container, the valve body member is moved against the spring force of the elastic means, with the result that the gap between the valve seat and the valve body member is increased and opens towards the bottom. Thus, the plug is released and the extract liquid can flow out downwards. By virtue of this design, a complete extraction of the coffee powder shall be ensured.
Thus, on the basis of the afore mentioned prior art, it is an object of the present invention to provide a coffee machine in which the user or operator can directly influence the amount of coffee beverage to be brewed per time unit.
It is a further object of the present invention to provide a coffee machine in which the brewing water flows through the coffee powder received in the brewing chamber with an essentially constant flow rate, independent of the amount of coffee powder received in the brewing chamber and the granule size thereof.
To meet these and other objects, the invention provides a coffee machine, comprising a fresh water supply, for example a water reservoir, a coffee brewing unit provided with a brewing chamber, means for supplying coffee powder to the brewing chamber, and a coffee beverage outlet. Further, a pump is provided for conveying water under pressure from the fresh water supply through the brewing chamber containing the coffee powder to the coffee beverage outlet. A valve assembly is inserted between the brewing chamber and the coffee beverage outlet whereby the valve assembly comprises means for adjusting the amount of coffee beverage flowing through the valve assembly per time unit.
Preferably, the valve assembly comprises a valve chamber and a valve body member movable in the valve chamber from an open position in which the coffee beverage can flow through the valve assembly to a closed position in which the flow of the coffee beverage through the valve assembly is blocked. Moreover, a spring acting on the valve body member is provided to bias the valve body member to move into the open position. The valve body member has a surface exposed to the coffee beverage flowing into the valve assembly, whereby the valve body member is adapted to be moved, against the biasing force exerted by the spring, into the closed position when the afore mentioned surface is charged by the pressure of the coffee beverage.
By the provision of means for adjusting the amount of coffee beverage flowing through the brewing chamber per time unit, the user or operator of the coffee machine can directly influence the flow rate of the brewing water flowing through the coffee powder contained in the brewing chamber and, thereby, the amount of coffee brewed per time unit. Since the valve assembly is designed as a regulating valve in which the spring is set to keep the valve body member in the open position, and since the valve body member can be moved under the influence of the coffee beverage flowing through the valve, against the force of the spring, towards the closed position, also the relative flow rate can be kept constant within certain limits. It is understood that the flow rate of the brewing water flowing through the coffee powder cannot be kept on an absolutely exact constant level by means of a valve assembly of the kind referred to herein, since the regulation behavior of the valve assembly depends, amongst else, on the viscosity of the liquid flowing there through. Thus, a varying viscosity of the coffee beverage flowing out of the brewing chamber can have, to a certain extent, an effect on the flow velocity of the brewing water flowing through the coffee powder contained in the brewing chamber.